Hoists for raising and lowering objects have been known since antiquity, and in more modern times hoist have been developed that may be attached to a vehicle. A boom capable of 360 degree rotation is described in U.S. Pat. No. 4,881,864 of Amata (1989). U.S. Pat. No. 5,029,740 of Cox (1991) describes construction features such as square or rectangular tubular structural members, concentrically cooperating with a trailer hitch, drawbar, hand operated connectors, and a frame permitting rollabout operation of the crane when it is not connected to a vehicle. U.S. Pat. No. 4,527,935 of Fortenberry (1985) discloses a trailer having an adjustable steadying and load supporting outrigger.
U.S. Pat. No. 5,211,526 of Robinette (1993) discloses a mobile crane attachable to a vehicle drawbar and other surfaces. The Robinette device provides a measure of adjustability. It has a telescopic boom, and two outriggers of adjustable height. However, it is not fully adjustable, in that the boom lifting means (such as a hydraulic jack) is fixedly attached to a support column and boom. There is no way to adjust the height of the support column, or the attachment position between the boom and support column. The Robinette device may be suitable for lifting and moving relatively light loads, but it could fail and possibly cause injury if it were attempted to be used to lift relatively heavy objects such as, by way of example, an automobile engine. The boom of Robinette has a "support member," but attaches to the central column at a simple through hole penetrating the boom, which could fail at high loads. Set screws secure the position of the crane against unwanted rotation to some extent, but they too could fail at high loads. Further, the connection between the lifting means and the central column places high stress on the column, and raises the center of gravity of the device. The rotation mechanism between the central column and receiver may well serve some purposes, but does not provide a tensioning force between the column and the receiver and consequently could fail, particularly when heavy loads are moving on the boom. The Robinette device is only described herein to illustrate the differences between the known prior art and the present invention described, which provides an adjustable crane of greater intrinsic strength, adjustability, and convenience of use, among other advantages, than has heretofore been known.